Southwest Airlines last month handed Baltimore-Washington International Airport its first significant gain in passenger traffic in nearly four years, as the number of passengers traveling through BWI jumped by nearly 100,000 from a year ago.

Southwest, a Dallas-based discount carrier, began flying Sept. 15 from BWI to Chicago and Cleveland. Although its traffic accounted for only 3 percent of BWI's 807,000 passengers in September, the upstart airline's debut forced down fares among other airlines, attracting droves of travelers.

The resulting 13.6 percent growth for September compares with a slight decrease in passenger traffic nationwide.

"Southwest and its fares certainly have stimulated the market," Theodore E. Mathison, administrator at BWI, said yesterday.

Since Southwest announced plans in July to begin service at BWI, the unrestricted one-way fare to Cleveland has dropped from $349 to$49. USAir cut its fares and added seven flights. Continental Airlines launched its Peanut Fares service from BWI to nine East Coast cities.

Southwest's decision to launch its first East Coast operation at BWI was heralded as one that would guarantee sizable increases in airport traffic. "It's not uncommon to see all the carriers gain traffic when we enter the market," Patrick Edwards, Southwest's sales manager at BWI, said yesterday.

Southwest also began service this year at San Jose, Calif., and Louisville, Ky. In Southwest's first month at Louisville, the number of passengers traveling to and from Chicago jumped from 8,000 to 23,000, according to Southwest.

In the two weeks in which it operated at BWI in September, Southwest carried just under 12,000 total passengers on its five daily flights to Cleveland and its five flights to Chicago. For October, the figure was 24,000, according to Mr. Edwards.

Mr. Edwards predicted that the steady traffic would continue at least through February, when the airline's promotional one-way flights of $19 to Cleveland and $39 to Chicago will end. The unrestricted one-way fares of $49 and $89 will continue.

Mr. Mathison said he expects overall traffic for October to increase as well, reflecting a full month of operation by Southwest, along with the lower fares by other airlines.

Last October, traffic was depressed by a five-day USAir machinists strike that affected many of the airline's flights.

In the spring, BWI experienced modest gains in traffic, after having suffered several years of industrywide declining traffic because of the Persian Gulf war and the recession. By summer, however, traffic was down slightly compared with the previous summer, when fare wars had sparked heavy travel.

Air cargo traffic was down about 10 percent, airport officials said, because of a stagnant economy and because KLM, a major overseas carrier, shifted its operation in May to Dulles International Airport near Washington. But the amount of air mail handled by carriers at BWI increased 10 percent.